So how would the Whelen perform on Grizzlies? Or would i be better off just getting a 375 or a 45-70 for that?

Here is a good article prepared several years ago by the Alaska game and fish dept. In a nutshell any of the magnums 375 and up worked best, but recoil levels are a concern. Gunwriters and professional guides Finn Aagard from Africa and Phil Shoemaker from Alaska both worked on similar tests. They all 3 concluded that short of a 375 mag a 30-06 loaded with 220 gr Nosler Partitions outperformed everything else tested. Including 35 Whelen, 338-06, and 45-70. While the various 338 mags had more energy, they couldn't match the .30's for penetration.